Method of securing fixing elements in rock

ABSTRACT

A method of securing a fixing element such as a dowel in a drillhole in a rock mass wherein the element is grouted in position with calcium sulphate hemihydrate plaster (preferably alpha gypsum) gauged with a solution of a water-soluble salt of carboxy methyl cellulose.

This invention relates to a method of securing fixing elements indrillholes in rock masses. The method is applicable for anchoring boltsor other rod like elements in rock and it is especially useful in longhole dowel reinforcement of rock in tunnels and coal mines. Theinvention also includes pumpable compositions for use in the method andthe method of preparing such compositions.

In the method of rock reinforcement which is widely practised in thecoal mining industry, several coupled dowels, usually of wood and ofsolid or hollow split-timber construction having a total length of abouttwenty feet, are inserted into predrilled holes into which a grout,usually of a synthetic hardenable resin composition, is pumped. Onsetting the resin binds the timber dowels into the surrounding rock. Inthis manner several weak strata may be knitted together into a strongercomposite beam. This technique is used for both tunnel roof and facesupport and has contributed to safer and more productive working of coalseams. A similar technique is employed to grout anchor bolts, forexample for the attachment of roof support plates and other fixtures inmines. In the normal methods the resin is injected around the fixingelement by pumping through a flexible loading tube.

Polyester resins and urea/formaldehyde (UF) resins have beensuccessfully used in this application. Polyester resins are muchstronger and would therefore be preferred to UF resins but they areinflammable and require chemicals to clean them from pumping equipment.UF resins in the ungelled state are easily soluble in water so that pumpcleaning presents no problem, and are not readily flammable. Howeverthey have the serious disadvantage that, on burning, they evolve toxicgases comprising appreciable quantities of hydrogen cyanide and thuswould be dangerous if they were involved in a fire at the working site.

It is an object of this invention to provide a method of grouting fixingelements in rock which does not involve the use of flammable orpotentially toxic grouting compositions.

A widely used hardening material is gypsum plaster based on calciumsulphate hemihydrate but this has not found favour for grouting fixingelements in rock in mines. Indeed the properties of such plaster asnormally used would be regarded as incompatible with the requirements ofa suitable pumpable grout. Thus gypsum plasters are considered to beweak materials and therefore unlikely to give the required bond strengthbetween the fixing element and the rock. They set too quickly to provideadequate time for pumping into position and, when formulated withsufficient gauging water for pumpability, they have impaired setstrength. In addition these plasters are difficult to pump uniformly,mainly because the viscosity in the unset state is unstable and tends toincrease rapidly and the unset plasters are not sufficiently thixotropicto prevent them from running out of upwardly inclined boreholes.

We have now found that calcium sulphate hemihydrate plasters containingwater-soluble salt of carboxy methyl cellulose (CMC) as set retardanthave a surprising combination of properties which render themparticularly suitable as pumpable grouting compositions for groutingfixing elements in rock. The CMC salt permits plasters of pumpableviscosity to be formulated with a reduced quantity of gauging water,thereby retaining the set strength of the plaster. Low viscosity,smooth, consistent plasters can be formulated which have less drag andcan be pumped surprisingly fast. The viscosity of the unset plaster canbe precisely controlled by a variation of the retardant concentrationand it remains low until near the setting time. An especiallyadvantageous property of these plasters is their surprisingly improvedthixotropy so that there is little flow after pumping is stopped andeasily pumpable compositions will stay in place in upwardly directedboreholes. A further advantage is that the slight expansion which occurson hardening of gypsum plasters is substantially maintained afterhardening, whereas gypsum plasters normally shrink a little over aperiod of a few days after initial hardening. This ensures that thefixing elements are quickly bonded tightly to the surrounding rock andremain firmly in position thereafter.

This beneficial combination of properties is conferred only by the CMCsalts and we have found that non-ionic cellulose ethers such as methylcellulose, methyl hydroxyethyl cellulose or hydroxypropyl methylcellulose, even is used in conjunction with protein retardant, do notconfer the same benefits, although they may be beneficial in improvingwater retention as described hereinafter.

In accordance with this invention, a fixing element is secured in adrillhole in a rock mass by grouting with a plaster composition based oncalcium sulphate hemihydrate gauged with an aqueous solution containing0.01 to 3% w/v (i.e. percent in grams per 100 cc of water) of awater-soluble salt of carboxy methyl cellulose (CMC).

The invention also includes the aforedescribed plaster composition foruse in the said method.

The grouting composition is preferably introduced into the drillholes bypumping and for easy pumping the grouting composition should preferablycontain 35 to 45 parts by weight of gauging water per 100 parts ofcalcium sulphate hemihydrate.

The preferred kinds of calcium sulphate hemihydrate plasters are thoseof low porosity, for example those termed alpha gypsum and described inKirk-Othmer, Encyclopedia of Chemical Technology, 2nd Edition, Volume 4Page 23. These plasters require a lower quantity of gauging water andhave higher set strengths than the more usual plasters, which are moreporous and absorbent, and in addition they give especially smooth,consistent, easily pumpable compositions.

The preferred grade of CMC is a low viscosity grade, of which a 1% w/vaqueous solution has a viscosity of 20 to 80 centipoises at 20° C. Thedegree of substitution is preferably in the range from 0.5 to 1.2carboxy methyl groups per glucose anhydride unit and preferably theMolecular Weight is in the range from 80,000 to 140,000. The preferredwater-soluble salt is the sodium salt. The CMC may be mixed with the dryplaster or the wet plaster but it is preferred to dissolve it first inthe gauging water and to gauge the plaster with the solution. Forconvenient working a hardening time of about 11/2 to 2 hours aftermixing of the plaster is desirable and this is achieved by using SCMC inamounts of 0.1 to 0.2% w/v (grams/100 cc) of the gauging water. If thegauging solution is to be stored before use it is beneficial to add, asa preservative, a biocide such as, for example benzisothiazolone.

The plaster composition may also include fillers in an amount up to 100parts per 100 parts of calcium sulphate hemihydrate by weight. Thefillers may comprise, for example, mineral clays, talc, blast furnaceslag, glass, asbestos, chalk, ground limestone, quartz, pumice, portlandcement, alumina cement, pozzolana cement, cellulosic fibre such aswoodmeal and sawdust, or synthetic plastics fibre such as nylon fibre.

The paster is readily prepared by mixing the ingredients together bynormal plaster mixing methods. The wet plaster slurry quickly developsthixotropy on standing but the original viscosity is easily restored bystirring. Some separation of water may occur on standing but any of thiswater which remains in contact with the plaster surface is reabsorbedduring hardening. This water separation is, however, a disadvantage ifthe plaster is to be used in absorbent strata because the water loss canresult in insufficient hydration of the hemihydrate with consequentimproper setting of the plaster. The water separation can be reduced orprevented by dissolving in the gauging water, from 0.01 to 5.0% w/v ofhydroxypropyl guar gum having a Molecular Weight in the range from500,000 to 5,000,000 or non-ionic cellulose ether, for examplehydroxypropyl methyl cellulose, methyl cellulose or methyl hydroxyethylcellulose.

The invention is further illustrated by the following Examples in whichparts and percentages are given by weight unless otherwise stated.

EXAMPLES 1-10

In the preparation of the plasters of these Examples 100 parts ofcalcium sulphate hemihydrate were gauged to a pumpable slurry with anaqueous solution of sodium carboxy methyl cellulose at 15° C. Details ofthe grades of calcium sulphate hemihydrate, the concentration and amountof gauging water, and the properties of the plasters are given inTable 1. In preparing the slurries the ingredients were mixed bystirring in a twin-blade paddle stirrer till a smooth consistent slurrywas obtained.

The α-gypsum was calcium sulphate hemihydrate produced by calcininggypsum in an autoclave in an atmosphere of steam and consisted ofroughly spherical non-porous particles of average diameter 0.5 to 1.5μ.

The calcium sulphate hemihydrate plaster used in Examples 9 and 10 was anormal absorbent plaster obtained by calcining gypsum at atmosphericpressure in a dry atmosphere and consisted of rod shaped particleshaving average length of 0.5 to 1.0μ and average diameter of 0.03 to0.3μ.

The SCMC used was a quick dissolving granular grade having a degree ofsubstitution of 0.7 and Molecular Weight of 80,000 to 140,000. In 1% w/vaqueous solution its viscosity was 50 centipoises at 15° C. Theparticles were of a size passing an 18 mesh BS sieve (0.855 mm) andretained on an 85 mesh BS sieve (0.18 mm).

Tensile strength tests were done 24 hours after initial hardening.

                                      TABLE 1                                     __________________________________________________________________________                Parts of                                                                      gauging                                                                Grade of                                                                             solution                                                                           SCMC in         Tensile                                           Calcium                                                                              per 100                                                                            gauging                                                                            Initial                                                                            Hardening                                                                           strength                                     Example                                                                            Sulphate                                                                             parts of                                                                           solution                                                                           viscosity                                                                          time  (kilo new-                                   No   Hemihydrate                                                                          α-gypsum                                                                     % w/v                                                                              (cps)                                                                              (minutes)                                                                           tons/m.sup.2)                                __________________________________________________________________________    1    α-gypsum                                                                       28   0.14 8820 87    2800                                         2    "      33.3 0.14 3908 100   3550                                         3    "      39   0.14 1224 115   3550                                         4    "      44   0.14 428  160   1450                                         5    "      50   0.14 288  300   1300                                         6    "      39   0.10 928  96    3100                                         7    "      39   0.12 1148 104   2800                                         8    "      39   0.16 1108 170   2800                                         9    Normal 39   0.14 3700 >450  3250                                         10   "      39   0.20 2040 210   3550                                         __________________________________________________________________________

The properties of the plaster given in the Table show that the optimumtensile strength is obtained using 33-39 parts of gauging solution. Theviscosity of the plaster slurry obtained using gauging solutioncontaining 0.10 to 0.15 w/v of SCMC is sufficiently low and thehardening time is sufficiently delayed for the slurry to be pumped intodrillholes as a grouting material for dowel reinforcement of rock. Theα-gypsum slurry of Example 3 had lower viscosity than the normal plasterslurry of Example 9 and the α-gypsum plasters were also smoother andmore consistent which also enhanced their pumpability.

Viscosity measurements were made on samples of the slurry of Example 3at 15° C., at various times after mixing, using a Brookfield Viscometer(RVT model) (with a number 5 spindle at 100 revolutions per minute androtating spindle for 2 minutes) and the following values were obtained.

                  TABLE 2                                                         ______________________________________                                        Time After Mixing   Viscosity at 15° C                                 (minutes)           (cps)                                                     ______________________________________                                        10                  1224                                                      20                  1352                                                      30                  1448                                                      60                  1280                                                      80                  600                                                       90                  1148                                                      100                 1680                                                      110                 1132                                                      115                 Plaster hardened                                          ______________________________________                                    

These measurements show that the viscosity remained substantiallyconstant almost until the plaster hardened. The set plaster had atensile strength of 3,550 kilo newtons/m² and a compressive strength of3,400 kilo newtons/m².

In a pumpability test the slurry of Example 3 was pumped through a 1.6cm diameter × 8 m long PVC pipe using a pump at 2,040 kilo newtons/m²delivery pressure at a rate of 29 kg per minute.

Some of the plaster was pumped into a 5 cm diameter × 2.5 m metal pipewhich was stoppered at one end and contained 2 coupled 2 m × 3.2 cmdiameter timber dowels hollowed out along their length to accommodate a1.6 cm diameter PVC delivery pipe. On setting, the plaster bonded thetimber dowel tightly to the metal pipe.

In a test of anchorage strength in concrete, a 20 mm diameter hightensile steel bolt was grouted in a 17.8 cm × 3.5 cm diameter hole in ablock of high density concrete (compressive strength 27,500 kilonewtons/m²) using the composition of Example 3 and pulled out by meansof a hand-operated hydraulic jack. The axial pull required was 66 kilonewtons, i.e. 3.7 kilo newtons/cm of bond. This compares favourably withan anchorage strength of about 4 kilo newtons/cm obtained with polyestercompositions and about 1.2 kilo newtons/cm obtained withurea/formaldehyde resin compositions.

EXAMPLE 11

100 parts of α-gypsum were mixed with 39 parts of aqueous gaugingsolution containing 0.10% w/v SCMC (as used in Example 1) and 0.05% w/vof hydroxy propylated guar gum till a smooth slurry was obtained. Thehydroxy propylated guar gum had a Molecular Weight of 3,000,000 and adegree of substitution of 1.0. A 2% aqueous solution had a viscosity of129,000 cps at 20° C.

There was no water separation from the slurry when it was allowed tostand before setting. The tensile strength of the set plaster was 35.50kilo newtons/m².

Viscosity measurements were made on samples of the slurry by the methodused in Example 1 and the values shown in Table 3 were obtained.

                  TABLE 3                                                         ______________________________________                                        Time after Mixing   Viscosity at 15° C                                 (minutes)           (cps)                                                     ______________________________________                                        10                  3376                                                      20                  3372                                                      30                  3592                                                      40                  16100                                                     50                  31760                                                     60                  62880                                                     80                  64480                                                     120                 Plaster hardened                                          ______________________________________                                    

The viscosity was maintained at a constant low value only for about 30minutes and rose rapidly thereafter.

EXAMPLE 12

100 parts of α-gypsum were mixed with 39 parts of aqueous gaugingsolution containing 0.15% w/v SCMC and 0.15% w/v of hydroxypropyl methylcellulose. The hydroxypropyl methyl cellulose had a Molecular Weight of60,000, a degree of substitution of 1.5 for methyl groups and 0.3 forhydroxypropyl groups and the viscosity of a 2% aqueous solution at 20°C. was 50 cps.

The variation of the plaster viscosity with time is shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Time after Mixing   Viscosity at 20° C                                 (minutes)           (cps)                                                     ______________________________________                                        10                  1580                                                      20                  2100                                                      30                  2240                                                      40                  2020                                                      50                  2540                                                      60                  2700                                                      70                  2800                                                      80                  3260                                                      90                  3980                                                      200                 Plaster hardened                                          ______________________________________                                    

EXAMPLES 13-19

In these Examples α-gypsum was mixed with various fillers and gaugingsolution as used in Examples 1 to 5, the amounts of filler and gaugingwater being adjusted so as to give slurries of viscosity 1,000 to 2,500cps at 15° C. Details of the composition and properties of the plastersare given in Table 5.

                                      TABLE 5                                     __________________________________________________________________________                Parts                                                                              Parts             Tensile                                                filler/100                                                                         solution/100                                                                         Initial                                                                            Hardening                                                                           strength                                   Example                                                                            Type of                                                                              parts                                                                              parts  viscosity                                                                          time  (kilo new-                                 No   filler plaster                                                                            plaster                                                                              (cps)                                                                              (minutes)                                                                           tons/m.sup.2)                              __________________________________________________________________________    13   China  30   65     1240 109   1250                                            clay                                                                     14   Power  30   45     1192 57    1550                                            Station                                                                       ash                                                                      15   Ground 60   50     1080 77    1850                                            limestone                                                                16   Talc   30   45     1416 76    1850                                       17   Bentonite                                                                            15   55     888  105   620                                        18   Portland                                                                             60   55     2440 38    1550                                            cement                                                                   19   Glass fibre                                                                          5    40     1220 84    2150                                            (6 mm long)                                                              __________________________________________________________________________

These results show that, whilst it is possible to add large quantitiesof filler without excessive reduction of the hardening time, the use ofthe filler increases the requirement of gauging water and markedlyweakens the set plaster.

EXAMPLE 20

In this Example the plaster composition of Example 3 was used in a coalmine as grouting material for wooden dowels inserted in drillholes toreinforce a faulted 1.53 m thick coal seam.

Two holes of 4.3 cm diameter spaced 92 cm apart were drilledhorizontally at right angles to the face to a depth of 7.3 m. Fourlengths of wooden dowel each 1.8 m long × 3.5 cm diameter, joinedtogether at the ends by step joints were inserted into each hole to lieflush with the hole mouth. The dowels were recessed along their lengthand a polythene slurry delivery tube having an external diameter of 1.9cm and an internal diameter of 1.59 cm, accommodated and secured byadhesive tape in the recess, extended along the length of the dowels toprovide a connection for injecting the plaster slurry. A further twoholes were drilled at right angles to the face and at an angle of 30°upwardly from the horizontal to a depth of 3.6 m from two points on theface positioned respectively 30.5 cm vertically above the previousholes. Two stepped end-joined recessed dowels identical to the dowelsdescribed above and also provided with a polythene delivery tube securedin the recess were inserted into each of the further holes.

Freshly prepared plaster slurry composition as prepared in Example 3 waspumped by a ram pump through the delivery tubes at a rate 4.5 to 9liters per minute until the space around each dowel was filled andslurry emerged from the hole mouth.

When the pumping was stopped there was negligible further exudation ofslurry from the holes. The protruding polythene tubing was cut away andthe holes were sealed with a small plug of stemming composition toprevent any displacement of the dowels or plaster. The plaster hardenedafter two hours and bonded the dowels firmly in position. The rock massaround the drillholes was thereby strengthened sufficiently to allow thecoal seam to be undercut and blasted in the usual manner.

What we claim is:
 1. A method of securing a fixing element in adrillhole in a rock mass comprising grouting the element by pumping intothe drillhole a plaster composition based on calcium sulphatehemihydrate comprising α-gypsum gauged with an aqueous solutioncontaining 0.01 to 3% w/v of water-soluble salt of carboxy methylcellulose (CMC), the grouting composition containing sufficient water torender the composition pumpable.
 2. A method as in claim 1 wherein thewater-soluble CMC salt is such that a 1% w/v aqueous solution has aviscosity of 20 to 80 centipoises at 20° C.
 3. A method as in claim 1wherein the CMC salt has a degree of substitution of 0.5 to 1.2.
 4. Amethod as in claim 1 wherein the CMC salt has a molecular weight in therange from 80,000 to 140,000.
 5. A method as in claim 1 wherein the CMCsalt is a sodium salt (SCMC).
 6. A method as in claim 5 wherein thegauging water comprises SCMC in amounts in the range from 0.1 to 0.2%w/v.
 7. A method as in claim 1 wherein the plaster composition comprisesfiller in an amount up to 100 parts per 100 parts of calcium sulphatehemihydrate by weight.
 8. A method as in claim 7 wherein the filler isselected from the group consisting of mineral clay, talc, blast furnaceslag, glass, asbestos, chalk, ground limestone, quartz, pumice, portlandcement, alumina cement, pozzolana cement, cellulosic fibre and syntheticplastics fibre.